地図作成

Input lag is the time it takes for a display to successfully get an image or frame from the moment the signal enters the display to your eyes. This is what causes the majority of the sync issues experienced with video games or voices on an LCD (or as this post will show, several other devices usually considered to be lag free). It’s not the same as refresh rate or response time. Refresh rate is how many times a second the display can draw a full image. Response time controls how long a single pixel changes color.

Response time

Response time used to be important with LCDs, but rarely do you find a new panel slow enough to be much of a problem anymore. Sure the twelve milliseconds where you can see part of both images after turning a corner might cost you a kill in UT2004 if you’re a tournament player, but us mortals don’t frag at such speeds.

The trailing image of the arrows is the ghosting caused by slower response times.

How much lag is too much?

If we measure the lag in frames at 60 per second, most will accept one frame as unnoticeable. Two frames will impact games, more will impact lip sync of voices. In one test, I was able to get a TV (the 42LGX below) to lag at nearly 250ms, or 15 frames, which just appears as though the TV is broken.

How to test

In the grand old days of electron guns pointed at a chunk of glass and phosphor, input lag was nearly non-existent. The easiest way to test is to clone the display from a computer to a CRT and the display in question, then start a timer and take pictures until you have a good sample size. In my tests I used an online stopwatch page and a Nikon D40 camera. The two reference displays were a CRT borrowed from a friend’s garage and the more portable Lenovo W500 LCD (TN panel). The W500 LCD was measured to be 0ms behind the CRT. All displays were tested at 60Hz.

An example test looks like this:

This is comparing our reference CRT with a Planar 720p DLP projector. This shot was rare, usually the Planar gave 2-3 frames of lag.

Some monitors compared

Planar Projector

Quick specs: DLP, 1280×720, DVI input
Median lag: 43ms or ~3 frames
Average lag: 40ms

Daewoo 17″ LCD

Quick specs: TN, 1280×1024, VGA input
Median lag: 43ms
Average lag: 30ms or ~2 frames

Acer 22″ LCD

Quick specs: TN, 1680×1050, DVI input
Median lag: 0ms
Average lag: Ghosting occurs, so partial frames are drawn, but never is anything more than the previous frame present.

Dell 30″ LCD – 3008WFP

Quick specs: S-IPS, 2560×1600 (1920×1200 here due to video card limitations), DVI input
Median lag: 42ms
Average lag: 42ms

Dell 30″ LCD – 3007WFP-HC

Quick specs: S-IPS, 2560×1600 (1920×1200 here due to video card limitations), DVI input
Median lag: 0ms
Average lag: 8ms

LG 42″ LCD HDTV – 42LGX

Quick specs: IPS, 1920×1080, HDMI input (DVI source)
Median lag: 43ms
Average lag: 45ms but varies greatly, lowest was 30ms, highest was 90ms. Enabling many of the image processing settings doubled the lag, making it annoying for watching TV.

Lenovo W500 WUXGA LCD – LTN154U2-L05

Quick specs: TN, 1920×1200, LVDS input
Median lag: 0ms
Average lag: 2ms

40″ Sony Bravia LCD HDTV – KDL-40W4100

Quick specs: PVA, 1920×1080, HDMI input (DisplayPort source)
Median lag: 43ms
Average lag: 35ms (~2-3 frames)

46″ Sharp Aquos LCD HDTV – LC46D65U

Quick specs: PVA, 1920×1080, HDMI input (DisplayPort source)
Median lag: 0ms
Average lag: 8ms (~0-1 frames)

47″ LG HDTV – 47LH30

Quick specs: IPS, 1920×1080, HDMI input (DisplayPort source)
Median lag: 0ms
Average lag: 8ms (~0-1 frames)

Conclusions

From the above medians/averages input lag was vastly different between displays. In the end, only three had performance I would say is good. The W500, the LG 47LH30, and the Sharp LC46D65U.

Results did not depend on panel technology, as is expected from a controller issue. IPS has been said to have lower lag, but the 42LGX was the slowest tested with its IPS panel. From the blurry/over-sharpened image, and wildly varying lag, it’s possible the 42LGX tested was damaged somehow. TN also had a slow performer, the Daewoo LCD (but as the second oldest display in the group, this should be expected).

Of the two Dell 30″ monitors, the extra scaler in the 3008WFP introduces a lot of lag. If you’re a gamer or particularly sensitive to input lag, the 3007WFP is a better choice. On the other hand 3008WFP is consistent and two-three frames is easy to adjust to in most situations and it has many more inputs.

Both 120Hz TVs tested were slower than their 60Hz cousins. This is due to the extra processing used to produce motion blur frames by interpolating the current and upcoming frame. As superior image processors become available it may become useful but the lag introduced was not worth the occasional smoothness in my opinion.

My final recommendation then is a Dell 3007WFP for your PC, and an LG 47LH30 or Sharp LC47D65U for your living room.